Integrated waste water treatment for reusage after cyanide type plating

ABSTRACT

USED OR WASTE WASH WATE TREATMENT SOLUTION FROM INDIVIDUAL CYANIDE TYPE PLATING LINES THAT APPLY COATINGS OF DIFFERENT METALS ON WORKPIECES, HAVING A CHELATING COMPOUND AND A DISSOLVED CONTENT OF PLATING METAL THAT HAVE BEEN PICKED UP FROM DRAG-OUT ON THE WORKPIECES, IS TREATED WITH A SALT OF METALS OF THE CLASS CONSISTING OF CALCIUM, BARIUM, LITHIUM, ALUMINUM AND MAGESIUM, THE SALT ACTS AS AN EXCHANGE ELEMENT IN THE CHELATING COMPOUND TO REPLACE A MORE NOBLE DISSOLVED METAL CONTENT IN THE WASH WATER TREATMENT SOLUTION BEFORE IT IS REUSED TO WASH-OFF DRAG-OUT FROM WORKPIECES BEING PROCESSED IN A DIFFERENT PLATING LINE.

United States Patent Oflice 3,682,701 Patented Aug. 8, 1972 3,682,701 INTEGRATED WASTE WATER TREATMENT FOR REUSAGE AFTER CYANIDE TYPE PLATING Leslie E. Laney, Ellwood City, Pa., assignor to Laney Laboratories Inc., Zelienople, Pa. Filed Feb. 12, 1970, Ser. No. 11,004 Int. Cl. C02b 1/18 U.S. Cl. 134-13 14 Claims ABSTRACT OF THE DISCLOSURE Used or waste wash water treatment solution from individual cyanide type plating lines that apply coatings of different metals on workpieces, having a chelating compound and a dissolved content of plating metal that have been picked up from drag-out on the workpieces, is treated with a salt of metals of the class consisting of calcium, barium, lithium, aluminum and magnesium. The salt acts as an exchange element in the chelating compound to replace a more noble dissolved metal content in the wash water treatment solution before it is reused to wash-01f drag-out from workpieces being processed in a different plating line.

This invention relates to the reconditioning of wash waters that have been used for removing cyanide plating bath drag-out from a metal plated surface of one metal and as reconditioned are to be used for removing dragout from a plated surface of a workpiece having a plating thereon of a different, less noble metal. An important phase of the invention deals with making a used wash water treatment solution fully innocuous for subsequent reuse, irrespective of whether or not the plated surface in the first usage is of different metal than or of a different nobility condition of the same metal used for the plated surface of the second usage.

A Waste treatment system for workpieces that has a toxic cyanide carryover or drag-out is dislcosed in US Pat. No. 2,725,314. It makes use of chlorine and a caustic for neutralizing an after-treatment wash water solution for the workpieces, such that a subsequent fresh water rinse will be innocuous for sewer or stream discharge and the treatment wash solution may be reused as neutralized. In the neutralization operation, cyanide compounds are oxidized to cyanate (NaCNO) and, ultimately, to the gases, carbon dioxide and nitrogen.

This treatment process has been highly successful, but a problem has arisen where more than one or a group of cyanide type plating lines for different metal coatings are being used in one plant. For example, the same finishing facility or plant may have a process line for Zinc plating using a zinc-cyanide plating system, a process line for cadmium plating using a cadmium-cyanide system, a line for copper plating using a copper-cyanide system, or a line for brass plating using a copper-zinc-cyanide system.

It is advantageous to collect the treatment waste water from a group of plating lines as a common mixed batch, to neutralize, reconstitute or recondition the batch for return to and reuse in the individual systems. That is, in more complex installations where, for example, a group or number of plating lines is being utilized for applying coatings of a different metal in each line, it is advantageous, both from the standpoint of saving of equipment and space, to combine the used wash waters, reconstitute them as a mixture, and apply them in accordance with the need to any or all of the lines.

Freshly plated metal surfaces, especially zinc or cadmium surfaces, are very active, and the wash solution has a high pH of about 10 to 14 and contains a relatively high concentration of chlorine, about 50 mg. to 4000 rng./l. The problem particularly arises by reason of the fact that chelating or sequestering agents have come into use as an ingredient of the make-up in plating solutions as a compound added to the proprietary plating solution mix. Such an agent is thought to improve the plating process by adding brightness and smoothness. Typical agents are ethylenediamine-tetraacetic acid (EDTA), nitrilotriacetic acid, ethyleneglycol-bis, and N,N-tetraacetic acid. I have determined the presence of such an agent causes a complexing of the metal content of the solution and prevents such content from being precipitated out from the solution. Plating metal is thus retained in soluble form in the solution, where otherwise, under the pH conditions of about 10 to 14, the metal salt or salts used in the plating operation in view of such pH and presence of no other solubilizing agent would be insoluble and precipitate. However, the chelating agent has the characteristic of holding plating metals in solution as a chelate thereof.

Where used wash water contains active chlorine as an oxidizing agent, it tends to accelerate an adverse reaction that has been found to occur between the chelated metal and the plated surface. where the chelated metal content of the wash solution is represented by a metal that is more noble than the plated metal. The reaction involves an electrochemical exchange of metals. This causes corrosion or tarnishing of the less noble metal and a plating out of the more noble metal. For example, cadmium in the Wash solution, being more noble than a zinc plating or coating, would react in such a manner.

The invention as devised is carried out in such a manner as to satisfy the chelating compounds hunger for holding a before-used plating metal in solution, where otherwise such metal, under the solution conditions involved, would precipitate as an inert material. This hunger is satisfied by providing or adding one of a class of metals of low nobility that will serve as an exchange medium with the plating metal that is chelated or held by the chelating agent, whether it is copper, cadmium, tin, zinc, nickel, silver or other more noble plating metal. Such exchange agent is provided in the used Wash solution in excess or over a stoichiometric amount to reduce the soluble more noble metal content to not greater than 4 to 5 mg./l. which represents the optimum maximum that is permissible without resultant adverse reaction with the plated surfaces of a workpiece.

The chelate-held metal or metal present in the actual plating solution are earlier soluble, since the cyanide content has held them in a complex, whereas Without the cyanide they would be insoluble. When the cyanide is eliminated by the reaction with the chlorine content of the wash water, the metals are complexed by the relatively small quantity of the chelating agent present, holding a sufiicient proportion or percentage of the plating metal or metals in solution as to give rise to the difli'culty. Although each of the metal salts of the class consisting of calcium, magnesium, barium, lithium and aluminum is suitable to satisfy a chelating agent, the particular salt that is an optimum for use will depend on thetype of chelating agent and particularly, on the type of metal that is being removed or deleted from the chelating complex. For example, it has been determined that copper in a complex can be exchanged better with magnesium; calcium is better for precipitating cadmium, although lithium and barium are also useful in this connection. However, calcium has been found to be an optimum ingredient for precipitating all the various plating metals.

As previously intimated, the exchange agent will be provided in excess in the used wash solution to provide a full or maximized exchange reaction with the chelated metal. Any excess does not interfere with the process but may be lost in settling out in the form of sludge. Generally speaking, the optimum maximized permissible level of the chelated metal is about 4 to 5 m-g./l. However, this tolerance level tends to vary with the type of plating metal. In general, the upper limit which at least minimizes the trouble caused by chelated metal is about to mg./l. By way of example, with copper the tolerance limit is not more than about 5 mg./l., while with cadmium the tolerance limit is in the 10 to 15 mg./l. range. Trouble from chelated copper may appear as low as about 3.56 mg./l. and will fully disappear below this level, while trouble with cadmium will fully disappear at or below about the 7 mg./l. level. The reference to noble and less noble metals is a designation based on the order of the metals in the electromotive series, an electrochemical tabulation that ranges metals in order of their tendency to react with water and acids, wherein a given metal has the property of displacing from solution those below it (or less nobility) in the series and is, in turn, displaced by those above it (of greater nobility) in the series.

The exchange metals are preferably added to the wash solution in soluble form, although they may also be added as a suspension of their insoluble salts. For simplicity of operation, the soluble form can be metered more easily. Also, it is advantageous to use the chloride of the salt, since the drag-out containing wash water has been chlorinated and consists mainly of chlorides in view of the chlorine reaction with cyanide.

Although a separate treatment tank for each of a group of plating system or lines may be provided for collecting the wash solution or recovery dip which is used to remove the drag-out plating solution from surfaces of the workpieces, it is desired to combine the treatment solutions in a single reservoir for pumping back to the individual treating wash or recovery dip baths or zones in the individual process lines. Initial separation is needed due to the presence of unprecipitated metals. Using an individual treatment wash station and providing a sufficiently large reservoir solution combining tank, it is possible to precipitate the various metals and settle them out before a solution is returned to each of the lines for reuse.

It has been determined that the presence of one or a combination of metals in such a reconstituted solution will not cause difliculty in a precipitated or suspended solid form, in that the insoluble metal compounds do not participate in electrochemical reaction from the standpoint that they are inert materials and do not carry an electrochemical charge. The cyanide ion is the only solvent ion for keeping such metals in solution. Thus, with an effective elimination of the cyanide compound, the solubility of such metals is eliminated as inert salts in the form of hydroxides or carbonates of the particular metal. It is thus apparent that the cyanide that constitutes the drag-out that is picked up by the washing solution of each plating line and which thus has to be conditioned, also influences the problem from the standpoint of the presence of dissolved metal salts in the solution.

One attempt has been made to meet the problem by reducing the quantity of cyanide in the actual processing line or system and substituting partially for the solvent action of the cyanide, representative chelating agents, such as EDTA, etc. It has been believed by those skilled in the art that the use of a chelating agent or agents will reduce the amount of cyanide that has to be decomposed with chlorination, thus reducing the chemical cost of waste treatment. Also, as previously mentioned, it has been believed that chelating agents tend to impart greater or enhanced brightness or luster to the plated surface. However, those skilled in the art have not realized the adverse efiect of a chelating agent from the standpoint of the problem above discussed.

It has been found that suflicient residual metal ions remain in a used treatment wash solution, as produced and reconditioned in the manner shown in FIG. 1 of US. Patent No. 2,725,314, even after the retention time provided by a final mixing reservoir tank, such as to interfere with trouble-free use of the reconstituted solution in washing the drag-out from surfaces of workpieces being processed in individual systems or lines, thus causing staining and discoloration of freshly plated metal surfaces. For example, in the finishing of zinc-plated workpieces or parts, relatively small quantities of dissolved or soluble cadmium or copper in the aqueous washing solution will cause a corrosive action on such plated surface, since both cadmium and copper are more noble than zinc. The same type of reaction will occur where the reconstituted wash solution contains soluble copper and is being applied for washing use in a cadmium plating system or line, and will also occur where the solution contains dissolved nickel and is being applied for washing use in a brass plating line. It has been determined that relatively small quantities of dissolved metal ions that are more noble or that lie above the particular metal being plated on the electromotive scale have an adverse effect on freshly plated surfaces of the workpiece.

By way of example, it was found that a first treatment solution maintained at a pH of about 13.1 and containing 1800 mg./l. of free chlorine and a dissolved copper concentration of about 3.56 mg./l. will produce an appreciable staining of the surfaces of workpieces that have been, for example, plated with zinc, aluminum or other less noble metal. Another or second solution which on analysis had a pH of 13, a free chlorine concentration of 2800 mg./l. and a dissolved cadmium concentration of 570 mg./l. caused objectionable results on, for example, a zinc-plated surface. Such contaminants are representative of the dissolved metal contents of typical operating lines. The first example represents a situation that occurs when both a zinc plating and a copper plating line or system are being used and the wash solution of such lines has been reconditioned and combined in a reservoir for individual reuse. The second example is illustrative of a situation where a plant employs a zinc plating line and a cadium plating line.

IIIl view of the above, it has thus been an object of the invention to provide a practical solution to the indicated problem, taking the various factors involved into consideration.

Another object has been to develop a treatment of Washing solutions used in cyanide plating systems or lines that will enable contaminated or used wash treatment solutions to be taken off, reconditioned and combined or combined and reconditioned, and reused indiscriminately as aqueous wash treatment solutions in any or all of the lines, without producing an adverse reaction on or staining previously plated surfaces of workpieces.

A further object of the invention has been to reconstitute or recondition metal cyanide containing used aqueous waste treatment solutions in such a manner that they can be most effectively reused for washing-off and removing drag-out on surfaces of workpieces in individual plating lines without damaging previously plated surfaces.

A still further object of the invention has been to provide means for making the metal content of a used wash water innocuous for subsequent washing utilization, particularly as applied to the washing of a surface that is coated with a dilferent metal.

These and other objects of the invention will appear to those skilled in the art from the description.

The drawing is an illustrative schematic of a treating line or system that may be employed in accordance with principles of the invention.

Briefly, the important discovery was made that the addition of salts, such as those of barium, lithium, magnesium, aluminum and particularly, as an optimum calcium salts, to a contaminated washing or treatment solution will overcome the difliculties and solve the problem by replacing the objectionable metal compound or compounds in the complex of the solution, allowing them to precipitate and thereby making aqueous wash treatment solution fully suitable for further use, with the elimination of the staining and tarnishing effects that heretofore have ensued.

In the first-mentioned exemplary treating solution, adding calcium chloride in the amount of about g./l. (1.55 g./l. of calcium content), with the pH and the chlorine concentration unchanged, the soluble copper concentration was reduced from about 3.56 mg./l. to 0.34 mg./l. After making an addition to the treatment solution of the second solution example of 1.5 g./l. of calcium chloride (Ca'Cl representing 0.465 g./l. of calcium, analysis showed an unchanged pH of 13, an unchanged free chlorine concentration of about 2800 mg./l. but with a soluble cadmium concentration of only about 7 mg./l. To further prove the experiment, 500 mg./l. of cadium in the form of cadmium cyanide was additionally added and, after replenishing the chlorine concentration consumed by the cadmium cyanide addition, and allowing an hours reaction time, the reanalyzed same solution was found to contain only 3.2 mg./l. of soluble cadmium in solution.

In investigating further, it was determined that a great variety of chelating agents or compounds are currently being used as addition agents to metal plating solutions, including those employed in zinc, cadmium, copper, brass and other plating processes and employed at a high pH level at which an in-line-cyanide chlorination system such as set forth in the above-mentioned patent can be used. Chelating compounds or agents, such as those previously specifically mentioned by way of example, have a preferential bonding action with lower nobility metals and especially with calcium. Such agents appear to have a tighter bond with the calcium ion, thus allowing precipitation of the cadmium, copper, nickel and other metal salts.

A factor existing in the desired reaction is the presence of relatively high concentrations of sodium carbonate such that calcium salts are added would normally be insoluble unless the chelating agents or compounds would solubilize them. It appears that the exchange reaction takes place either with precipitated calcium or other metal carbonate or with the soluble calcium or other metal salt before it is precipitated from the wash water or solution. It has been found that sodium of potassium carbonates are always present in such waste treatment solutions in view of the fact that a cyanide type plating bath are operated at a high pH, as attained by use of caustic soda or caustic potash. Both the latter chemicals undergo some decomposition to carbonates taking carbon dioxide from the air. Also, the cyanide chemicals are continuously partially oxidized to carbonates as one of the final breakdown products of a cyanide compound.

In carrying out the invention, a metal displacing salt of the class consisting of calcium, barium, lithium, magnesium and aluminum is applied in an excess quantity to a used aqueous treatment wash or waste solution of a typical content that contains a chelating agent to exchange dissolved salts of metals, such as those of copper, cad mium, nickel, silver and lead. The exchange is accomplished by precipitating, making inert or dissolving-out such more noble metal contaminants by using the calcium or other less noble metals as an innocuous exchange substitute therefore in the solution.

The treatment processing of the invention may be applied either to individually taken-off and collected washing solutions from different plating systems or lines before such solutions are combined in a reservoir, but preferably at the time they are combined, so that the exchange may be all inclusive as to the mixed wash Waters to make them fully reconditioned for reuse in any or all of the plating lines or systems, indiscriminately without danger of corroding, damaging or discoloring plated surfaces of workpieces moving along such systems or lines. Although the specific requirements as to the quantity or amount of exchange chemical can be more easily calculated from the standpoint of its application to an individual wash solution that contains a specific type of metal contaminant, no adverse results are encountered in adding an excess of exchange salt to a mixed wash solution containing several types of metal contaminants. In other words, for example, if calcium chloride is being used, suflicient of it will be provided in the treatment solution reservoir or tank -E (see the drawing) to avoid going below the stoichiometric requirements of a desired exchange operation.

Utilizing the invention, a used wash treatment solution containing alkali metal hydroxides and alkali metal carbonates and cyanide treatment carryover residue of a cyanide plating bath having a pH within a range of about 10.5 to 14 which has been treated with free chlorine, results in the presence of an alkaline metal hypochlorite in the amount of at least 50 mg./l. The calcium or other exchange salt may be added either continually or periodically to the taken-off used solution to effect transition of the dissolved metal compounds into insoluble metal compounds. This results in the precipitation of such metal salts from the solution.

In the arrangement of the figure, A illustrates the last treating tank in a work process line where, for example, a cyanide solution is employed. The work line 10 may be a continuous strip denoting the travel path of the work process line, with tank B serving as a recovery dip for the cyanide solution which is returned to the tank A through line 11. In tank C, the work line or piece 10 is shown as being given a toxic carryover treatment wash and, in tank D, as being given a fresh water rinse. Overflow 14 controls the level of solution in tank C and has a branch line 15 returning the solution to the reservoir B through a header 35 and main line 15a. A liquid chlorine container F is provided to, with the solution of container G, renew the solution in container or reservoir E. Water may be continuously supplied to the rinse tank D from a source 16 and discharged to a sewer through line 17. Overflow 18 maintains the level of solution in the tank and may also discharge into the sewer through line 19. It will be noted that main return flow line 21a is connected to an overflow return 20 of the reservoir E and that the solution moves along the main return line 21a as aided by pump 34 into a common header 36 and from thence into at least one branch return line, such as line 21. Chlorine from liquid container F may be supplied by line 26 where it is fed to control valve 27 of a chlorinator 27a as a gas and out through a recorder 27b, connector 28 to connector 24 to discharge through lines 25 and 29 into the reservoir E. Although main outflow line 22 which removes used solution from the tank C through the agency of the pump 23, is shown as applied only to one representative treatment wash tank C, it will be apparent that it, like the outflow line 15, may be connected by a header (not shown) with branch lines from other treatment wash tanks C of other processing lines (not shown).

The present application fully incorporates the disclosure of the Lancy U.S. Pat. No. 2,725,314, including the showing of the FIG. 1 diagrammatic layout. The only change in such a layout needed (see the drawing of the present application) to carry out the present invention is to use a solution container corresponding to G for a less noble metal salt, such as for a calcium, lithium, barium, magnesium or aluminum chloride, and to employ a pump 30 in a line, such as 31, for introducing the salt in solution form into treatment solution reservoir E. If the container G for such salt is, however, located above the reservoir E, gravity flow may be employed. Thus, in accordance with the present invention, used treatment wash solution, such as from a tank or bath C of each of a group of lines for applying different plating metals, may be taken-off, as by lines 15, 15" and 22 leading from the treatment wash tank (corresponding to C) of each line, and introduced into a treatment reservoir E. It is preferable to employ a single reservoir E as a treatment reservoir for a mixture of the washing solutions taken from the group of workpiece treating lines. The overflow return shown may be connected to a common header 36 of line 21a for a group of return lines 21, 21 and 21", one for the treatment washbath or tank C of each individual line of the group of lines for plating different metals. If the tanks or baths C are located at higher levels than the reservoir E, then a suitable return flow pump corresponding to the pump or a suitable centrifugal pump 34 may be utilized in the common outflow header for the main return line 21a or an individual pump may be used in each branch line 21, 21' and 21".

It will be apparent that the treating or reconditioning of used wash treatment solutions from one or a group of treatment wash baths, zones or tanks, such as C, may be continuously effected in accordance with the procedure, with a continuous return flow of reconstituted or reconditioned solution being supplied to each tank. By using a metal salt of low nobility, such as calcium metal, the salt may be employed fully for all of the used solutions, irrespective of the more noble metal that is being plated in a particular electroplating line. If, however, it is desired to use a metal replacement salt for the treatment of the wash solution of one treating line which salt has a greater nobility than the plating metal of a second plating line, then the second plating line may have a separate treat ment reservoir similar to E for therein reacting its replacement salt with the chelated metal and for providing a full reconditioning treatment using the chlorine and caustic reactants. The base metal salts that are employed as exchange ingredients for chelated plating metal may be employed in the used treating solution either singularly or as a mixture of one or more of the specified class of less noble metals. {Whether one or more metal salts are used, the criterion is that all of them should be of a less noble metal than the plating metal that is to be removed from a dissolved condition in the used solution.

I claim:

1. In a method of reconditioning waste wash water solution after it has been applied to plated surfaces of a workpiece and has picked up drag-out therefrom that has been carried over from a cyanide metal plating bath and contains a chelating compound and plating metal dissolved therein, the improvement comprising introducing and excess of an exchange ingredient into the waste wash water solution of a salt of a less noble metal than the dissolved plating metal, reacting the ingredient and replacing the dissolved plating metal in the solution therewith while precipitating-out the plating metal, and after chemically enriching the waste wash water solution with chlorine and an alkaline metal hydroxide, again applying it to surfaces of a workpiece that has been plated in a cyanide metal plating bath.

2. In a method as defined in claim 1 wherein the exchange ingredient introduced into the wash water solution comprises a salt of a metal of the class consisting of calcium, lithium, barium, aluminum and magnesium.

3. In a method as defined in claim 1 wherein the exchange ingredient introduced into the wash water solution is a calcium salt.

4. In a method as defined in claim 1 wherein the plating metal is retained in soluble form by the chelating compound, employing the exchange ingredient when introduced into the waste wash water solution to replace the plating metal in the chelating compound.

5. In a method of reconditioning an aqueous waste treatment solution that has been removed from an in-line metal plating line after having been applied to Wash-off and receive drag-out from surfaces of a metal workpiece after a metal coating has been applied thereto in a cyanide treating bath, and wherein the waste solution contains chlorine and a chelating compound; the improvement comprising introducing an exchange metal salt into the solution and reacting it with the chelating compound to replace plating metal held in soluble form thereby, the exchange metal salt being of a metal that is less noble than the plating metal, and precipitating the replaced metal in insoluble form, all as a step in reconditioning'the aqueous solution for reuse in application to surfaces of plated metal workpieces. 6. In a method as defined in claim 5 wherein the aqueous waste treatment solution is collected, recondi tioned, and reused; the reconditioning being effected by employing neutralizing chemicals in the solution for neutralizing the drag-out taken from the surfaces of the workpiece, and by adding the exchange metal salt to the solution after the solution has been collected and before it is reused.

7. In a method as defined in claim 6 applying the solution as reconditioned to surfaces of a workpiece having a different plated metal thereon than in its first defined application.

8. In a method as defined in claim 5, providing the solution with a neutralizing chemical content when applied to surfaces of the workpiece, after the solution has been applied to the workpiece, removing it into a collecting zone and thereat adding a metal salt that is less noble than the metal carried in a soluble form in the solution from the drag-out, replacing the soluble metal in the solution with the less noble metal salt, thereafter returning the solution to the plating line for application to the surface of a workpiece that has been plated with a metal of a greater nobility than the nobility of the exchange metal salt.

9. In a method as defined in claim 8, providing an excess of the metal salt of lesser nobility in the solution.

10. In a method as defined in claim 8 wherein the pH of the solution taken-ofit' in used condition is within the range of about 10 to 15 and chlorine and an alkaline metal are added thereto to enrich it for reuse, the less noble exchange metal salt is of the classconsisting of calcium, lithium, barium, aluminum and magnesium, and the plated surface of the workpiece is of the class consisting of tin, copper, brass, cadmium, silver and nickel.

11. In a method as defined in claim 5 wherein a group of cyanide plating lines is being employed and each plating line applies a different plating metal to workpieces being moved therealong, applying an aqueous neutralizing chemical solution to the workpieces passing from a cyanide metal plating bath of each line to wash-01f and react with drag-out on each of the workpieces, thereafter removing the used solutions from each of the lines and collecting them, and chemically reconditioning the solutions in the collecting zone including adding an excess of a metal salt thereto that is of a less noble metal than any of the plating metals, and converting soluble plating metal contents of the collected solutions into insoluble metal by exchanging the less noble metal saltwithin the solution. a

12. In a method as defined in claim 11, after enriching and fully reconditioning the collected solution by introducing chlorine and an alkaline metal hydroxide thereto, then applying the solution indiscriminately to plated metal 10 surfaces of the group of lines for washing 0E and con- References Cited gigggllagchdggilcultingzmed over by workpieces moving UNITED STATES PATENTS 13. In a method as defined in claim 12, removing 212,890 5/1877 Brochocki 252 103X precipitated plating metals from the collecting solutions to 5 2,725,314 11/1955 Laney 134 13 limit the solution to a maximum content of such plating metals in soluble form within a range of about 10 to 15 MORRIS WOLK Primary Examiner mg/l. D. G. MILLMAN, Assistant Examiner 14. In a method as defined in claim 5, limiting the level of retained soluble plating metal within the solution 10 US. Cl. X.R.

to within a range of about 4 to 5 mg./l. 21060, 62 

